Electric arc discharge apparatus with a liquid cathode



Jan 28, 1964 J. H. HoLLlDAY ETAT. 3,119,943

ELECTRIC ARC DISCHARGE APPARATUS WITH A LIQUID cATHoDE Filed Nov. 1e, 1961 /1/ l1 llll FIT-FORM 'SYS United States Patent O 3,119,943 ELECTRIC ARC DISCHARGE APPARATUS WITH A LIQUID CATHODE John Hubert Holliday, Wembley Park, Terence Alexander Ketteringham, Pinner, Roy Crutchley Newbold, Harrow, and Arthur Jerrard Booth, Watford, England, assignors to The General Electric Company Limited, London, England Filed Nov. 16, 1961, Ser. No. 152,753 7 Claims. (Cl. 313-29) This invention relates to electric arc discharge apparatus.

In arc discharge apparatus comprising a pair of main electrodes within an evacuated envelope, the current carrying capacity of the apparatus depends upon the availability of gas or vapour ions within the envelope; thus in a mercury arc rectier mercury vapour can be produced in great quantities from the cathode pool and such a rectifier is capable of carrying very high currents. On the other hand if the vapour or gas pressure within the envelope is greater than a certain amount the presence of the gas or vapour limits the voltage diiference which can be applied between the main electrodes of the apparatus. Thus the resistance to voltage breakdown is greatest in apparatus in which the main electrodes are Separated by a vacuum gap, that is to say a space in which the vapour or gas pressure is such that the resistances to voltage breakdown is substantially independent of this pressure.

One object of the present invention is to provide an electric discharge apparatus capable of carrying high currents of short duration at least, yet capable of withstanding high voltage differences between its electrodes when the apparatus is not conducting.

A further object of the invention is to provide an irnproved igniter device for use in such apparatus.

According to the invention, the apparatus comprises an anode electrode and a cathode electrode mounted within an evacuated envelope and means for initiating an arc discharge between said electrodes, said cathode electrode comprising a iilm of liquid metal upon a solid cathode member, and said means comprising a pair of sparking electrodes extending down towards the lm and forming a spark gap in close proximity to said film.

In one embodiment of the invention the said means comprise a pair of coaxial conductors insulated from one another and forming an annular spark gap between the ends thereof, said spark gap being disposed in close proximity to said lilm.

In order that the invention may be clearly understood, one embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which FIGURE l shows a sectional elevation of the apparatus, and FIGURE 2 shows an enlarged section of a detail in FIGURE l.

Referring to the drawings, an electric arc discharge apparatus suitable for the conduction of current pulses of high intensity and short duration comprises an anode electrode 1 and a cathode electrode 2 mounted within an evacuated envelope 3, and an igniter device 4 for initiating an arc discharge between said electrodes. In operation, the anode electrode is made positive With respect to the cathode electrode and the current pulses are produced by pulsing the igniter device 4.

The envelope 3 is cylindrical and of metal, and the anode 1 is mounted coaxially within it and insulated therefrom by suitable metal to glass seals 5. The anode electrode 1 comprises a composite bulbous anode member constituted by a ilattened dish 18 of molybdenum and a tubular member 11 secured to the rim of the dish, the lower surface of the dish 18 constituting a planar anode 3,119,943 Patented Jan. 28, 1964 operating surface. The composite member is mounted at one end of a cylindrical tubular stem 6 constituting an anode lead. The anode is arranged to be cooled internally by the circulation of coolant via coolant connections 14 and 15. Surrounding the anode lead and coaxial with it is a second cylindrical tubular stem 7 constituting a cathode lead, this second stem being conductively secured to the envelope by ilange, 30 and 31 which are bolted together.

Since the apparatus is designed to carry large currents of short duration it is important to keep the inductance of the leads 6 and 7 to a minimum, and at the same time they have to be capable of withstanding the high potential difference applied between them. To keep the inductance as low as possible the ratio of the radii of the tubular stems should be as nearly unity as possible, but this would be inconsistent with the requirement of high resistance to voltage breakdown. In practice a cornpromise has to be found. Furthermore, in order to reduce the electrical stress at the sides of the anode as far as possible, the anode is shaped in the manner shown and a shield member 13 at cathode potential is disposed around the anode. The shield member is conductively secured to the envelope, and is spaced from the anode by a distance corresponding to the anode-cathode spacing so as to maintain a substantially uniform electric field around the anode.

The cathode electrode 2 comprises an inverted cupshaped or hollow member S of arc resisting metal, such as molybdenum, which is conductively secured to the lower end of the envelope, the member having a planar upper surface which is adjacent and parallel to the anode operating surface. Surrounding the member 8 is an annular pool 9 of mercury, which readily wets the surface of the member 8 to form a thin film of mercury extending over the surface of the cathode member. The lower, or concave surface of the member 8 is arranged to be cooled by cooling liquid during operation of the apparatus, coolant connections 32 and 33, and a baffle member 34, being provided for this purpose.

The igniter device 4 comprises a pair `of coaxial electrodes 20 and 21 projecting in from the side of the cylindrical envelope 3 and extending down towards the mercury lm on the cathode member. The inner electrode 2t) is of tungsten and is insulated for the greater part of its length by a glass sheath 22. The outer electrode 21 is apertured at its lower end to provide an annular spark gap 23 between this end of the outer electrode and the unsheathed end of the inner electrode 20, this spark gap being in close proximity with the cathode mercury film. The two electrodes are connected together in an insulating manner by a glass seal 24 disposed towards the upper end of the assembly, and the assembly is iitted with a flange 25 arranged to co-operate with the ilange 17 for fitting it into the apparatus. A terminal 26 is provided for connecting a suitable lead to the electrode 21, and a terminal connection 37 is provided for the inner electrode 20.

In operation the vapour pressure within the envelope is maintained at a low value, by cooling of the cup-shaped member 8 and by the envelope 3 itself if necessary, such that resistance to voltage breakdown between the anode 1 and the cathode 2 when a potential difference is applied therebetween is as great as if the apparatus were a vacuum discharge device. In order to establish an arc discharge, the anode is made positive with respect to the cathode and a high voltage pulse, of the order of say 50 kv., is applied to the igniter device causing an are to be struck to the cathode iilm. The mercury of the lm is thereby rapidly vaporised and ionised, whereupon the apparatus acts as a mercury vapour discharge device unice 3 til such time as the anode to cathode voltage is reduced to zero.

The anode should not be made positive again until such time as the conditions within the envelope are again those of a vacuum, the mercury vapour having again condensed and returned to the cathode. The apparatus described is suitable for the production of current pulses of about a millisecond duration at, say, ten second intervals, the current rising to a maximum value of 150,000 amps.

For operating the igniter device, a large condenser (not shown) is connected between the terminal 26 and the cathode, so that the potential of the outer electrode 21 is held down to the potential of the cathode, and the arc discharge between the main electrodes of the apparatus is initiated by applying the high voltage pulse to the electrode 20 s0 as to establish a spark across the gap 23; the spacing of the gap from the cathode is such that the vapour ions created by the spark are sufficient t cause the main discharge to build up.

The effectiveness of this method of ignition is considerably increased if the condenser is connected across a D.C. supply for when a cathode spot is formed this condenser then discharges and firmly establishes the cathode spot so that the main discharge can rapidly build up.

It will be seen that with this construction the spark gap between the igniter electrodes is situated away from, and shielded from the heaviest part of the main arc discharge during operation of the device, and being spaced from the cathode member it is not liable to become shortcircuited by the condensation of vaporised metal or damaged by the main discharge.

In another modification, a number of tubular anode leads connected in parallel are provided, each being coaxial with a surrounding cathode lead. The overall inductancc of the leads is reduced according to the number of parallel current paths.

In yet another modification, the anode and cathode leads comprise coaxial cables having tubular inner con ductors. The outer conductors, forming the cathode leads, are conductively secured to the envelope, and the inner conductors are extended parallel to the axis of the envelope and secured to the tubular stem of the anode at its upper end. These inner conductors are spaced around the anode stem forming a squirrel cage, the arrangement being such that the current in the tubular conductors flows in the opposite direction from that in the anode stem to provide a substantially non-inductive arrangement.

We claim:

l. Metal vapour electric arc discharge apparatus comprising an anode electrode and a cathode electrode mounted within an evacuated envelope and means for initiating an arc discharge between said electrodes, said cathode electrode comprising a film of liquid metal upon a solid cathode member, and said means comprising a pair of sparking electrodes extending down towards the film and forming a spark gap in close proximity to said film.

2. Metal vapour electric arc discharge apparatus comprising an anode electrode and a cathode electrode mounted within an evacuated envelope and means for initiating an arc discharge between said electrodes, said cathode electrode comprising a film of liquid metal upon a solid cathode member, and said means comprising a pair of coaxial conductors insulated from one another and forming an annular spark gap between the ends thereof, said spark gap being disposed in close proximity to said film.

3. Metal vapour electric arc discharge apparatus comprising an evacuated envelope, an anode electrode and a cathode electrode mounted within said envelope, each said electrode having an operating surface and said operating surfaces being parallel to, and spaced from, one another, a pool of liquid metal in contact with said cathode electrode, a film of liquid metal extending over the operating surface of said Cathode electrode, and means for initiating an arc discharge between said electrodes, said means comprising a pair of sparking electrodes eX- tending towards said film and forming a spark gap therebetween in close proximity to the film.

4. Metal vapour electric arc discharge apparatus comprising an evacuated envelope, an anode electrode and a cathode electrode mounted within said envelope, each said electrode comprising a hollow electrode member with a circular operating surface, said operating surfaces being disposed parallel to one another and spaced apart by a distance less than the radius of each said surface, a pool of liquid metal surrounding said cathode member, a film of liquid metal extending over the operating surface of said cathode member, means for initiating an arc discharge between said electrodes, said means comprising a pair of sparking electrodes extending towards said lm and forming a spark gap therebetween in close proximity to the film, means for circulating coolant internally of the anode member for cooling said member, and means for circulating coolant internally of the cathode member for cooling said cathode member.

5. Metal vapour electric arc discharge apparatus comprising an evacuated envelope, an anode electrode and a cathode electrode mounted within said envelope, said cathode electrode comprising a cathode member mounted at one end of said envelope and having an operating surface, and said anode electrode comprising a hollow bulbous anode member depending from the other end of said envelope and having an operating surface disposed adjacent and parallel to the operating surface of said cathode member, a pool of liquid metal in contact with said cathode member, a film of liquid metal extending over the operating surface of said cathode member, a shield member surrounding said anode member and conductively secured to said envelope, said shield member conforming to the shape of said bulbous anode member and being spaced therefrom by a distance corresponding to the spacing of said operative surfaces, for maintaining a uniform electric field around said anode member, and means for initiating an arc discharge between said electrodes, said means comprising a pair of coaxial conductors insulated from one another and forming an annular spark gap therebetween, said spark gap being disposed in close proximity to the film.

6. A metal vapour electric arc discharge apparatus comprising an anode and a liquid cathode mounted within an evacuated envelope, an igniter device including a pair of coaxial conductors arranged one within the other and forming between their ends an annular spark gap and i11- sulating means for securing said conductors in predetermined relationship, and means for mounting said device in the envelope above the liquid cathode and so that said annular spark gap lies in close proximity to the liquid cathode.

7. Metal vapour electric arc discharge apparatus comprising an evacuated cylindrical envelope, first and second main electrodes arranged coaxially within said envelope, said first main electrode comprising a hollow cathode member mounted at one end of the envelope in conductive relation thereto and having a substantially planar operating surface, and said second main electrode comprising a hollow bulbous anode member depending from the other end of the envelope and having a substantially planar operating surface disposed adjacent and parallel to the operating surface of said cathode member, a pool of liquid metal surrounding said cathode member, a film of liquid metal extending over the operating surface of said cathode member, a shield member surrounding said anode member and conductively secured to said envelope, said shield member conforming to the shape of the said bulbous anode member and being spaced therefrom by a distance corresponding to the spacing between said main electrodes for maintaining a uniform electric field around said anode member, means for initiating an arc discharge between said main electrodes comprising a pair of coaxial conductors insulated from one another and forming an annular spark gap therebetween, said spark gap being disposed in close proximity to said film, means for circulating coolant internally of said main electrodes, terminal connections to said coaxial conductors, and means providing electrical connections to said main electrodes, said means comprising a pair of coaxial tubular conductors arranged one Within the other and conductively secured to the anode member and the envelope respectively.

References Cited in the le of this patent UNITED STATES PATENTS Widmer Apr. 18, 1933 Cox u- Sept. 8, 1942 Hansell Nov. 18, 1947 Herring Mar. 23, 1954 FOREIGN PATENTS Great Britain Oct. 2, 1957 

4. METAL VAPOUR ELECTRIC ARC DISCHARGE APPARATUS COMPRISING AN EVACUATED ENVELOPE, AN ANODE ELECTRODE AND A CATHODE ELECTRODE MOUNTED WITHIN SAID ENVELOPE, EACH SAID ELECTRODE COMPRISING A HOLLOW ELECTRODE MEMBER WITH A CIRCULAR OPERATING SURFACE, SAID OPERATING SURFACES BEING DISPOSED PARALLEL TO ONE ANOTHER AND SPACED APART BY A DISTANCE LESS THAN THE RADIUS OF EACH SAID SURFACE, A POOL OF LIQUID METAL SURROUNDING SAID CATHODE MEMBER, A FILM OF LIQUID METAL EXTENDING OVER THE OPERATING SURFACE OF SAID CATHODE MEMBER, MEANS FOR INITIATING AN ARC DISCHARGE BETWEEN SAID ELECTRODES, SAID MEANS COMPRISING A PAIR OF SPARKING ELECTRODES EXTENDING TOWARDS SAID FILM AND FORMING A SPARK GAP THEREBETWEEN IN CLOSE PROXIMITY TO THE FILM, MEANS FOR CIRCULATING COOLANT INTERNALLY OF THE ANODE MEMBER FOR COOLING SAID MEMBER, AND MEANS FOR CIRCULATING COOLANT INTERNALLY OF THE CATHODE MEMBER FOR COOLING SAID CATHODE MEMBER. 